Back in the 18th century, German physician Franz Mesmer peddled a concept called animal magnetism. Creatures contain a universal fluid, he asserted, that when blocked in flow, caused sickness. Mesmer used magnetized objects to redirect that flow in patients, initiating unusual body sensations, fainting, vomiting or violent convulsions that ended in profound salubrious effects.

Skeptical, Benjamin Franklin and French chemist Antoine-Laurent Lavoisier simulated one of Mesmer's typical sessions in 1784. They asked people suffering from ailments ranging from asthma to epilepsy to hug “magnetized” trees. The people swooned and shook, as expected. But then the researchers divulged that the trees were never magnetized. And everyone realized that something else was inducing the reactions to the trees. That something was later dubbed the placebo effect.